Photon sampling technology has played an important role in high-speed signal processing and conversion, high-resolution measuring equipment, and optical signal quality testing. At present, high-performance photon sampling technology is at the stage of rapid development. The two major developing trends are ultra-high sampling rate and ultra-high precision. Considering from the aspect of the high sampling rate, the WDM/TDM scheme of the optical clock of the mode-locked laser can realize the multiplication of sampling rate, improve the sampling rate, and has the characteristics of strong stability, low clock jitter, and low electric processing quantization rate, so it is regarded as the best scheme of photon analog-to-digital conversion. In the current reported studies, the passive mode-locked laser is generally selected as the seed light source due to its low noise. However, the passive mode-locked laser has a low repetition frequency, and the acquisition of the high-rate photon sampling clock requires more multiplexing channels, which often leads to large structure and more stringent requirement for the precision of channel matching. With the development of the active mode-locked laser technology, the noise of active mode-locked lasers has been able to be reduced to a lower level. Using an active mode-locked laser with low jitter as the light source, based on its advantage of high repetition frequency, a photon sampling clock with high quality and ultra-high speed can be obtained only through a few multiplexing channels, which is of great significance for improving the performance index of the optical to digital conversion system and optimizing the system scheme.
However, clock jitter is a significant factor that limits the accuracy of photon sampling. Therefore, when improving the performance of a photon sampling system, the problem it faces is how to reduce the clock jitter between the photon sampling clock and the sampled signal source. In order to eliminate the relative clock jitter between the photon sampling clock and the signal to be sampled, it is necessary to improve the coherence between the two. One of these techniques is based on the same highly stable light source to simultaneously generate a coherent signal and a sampling clock, and the PADC resolution limit at this time will depend on the clock jitter of the light source itself. However, in practical applications, the broader case is that both the signal to be sampled and the sampling clock are generated from different signal sources.
Therefore, there is a need to realize high performance coherence between different electron and photon signal sources. Phase-locked technology is an effective means to realize coherent. By locking the frequency and phase of the controlled signal and the reference signal, their frequency and phase remain fixed, which reduces the clock jitter and improves the stability of the system.
Coherent phase-locked technology mainly includes the following. The first one is a photoelectric phase-discriminated and phase-locked technology that is based on optical nonlinear effects (J. Kim et al., “Drift-free femtosecond timing synchronization of remote optical and microwave sources,” Nature Photonics, 2008, 2: 733-736), where a variety of nonlinear optical crystals have been developed, and the crystal with second-harmonic generation effect (SHG) and sum frequency effect has great application prospects in optical phase detection. In a long-distance optical fiber transmission system, a photon phase discriminator composed of a crystal having a second-harmonic generation effect is used to measure the phase shift between the signals at the sending and receiving ends and feed it back. The photon phase discriminator and the photoelectric locked system based on the frequency crystal (See J. Kim et al., Nature Photonics, 2008, 2: 733-736) adopt an all-fiber structure, the stability of the system is high, and the phase discriminator adopts a balanced structure, which effectively eliminates the noise introduced by the channel imbalance. However, the phase-locked technology based on nonlinear crystal has obvious shortcomings. The system structure is complex and difficult to integrate. At the same time, the performance and stability of the nonlinear crystal are greatly affected by the environment, which limits the applicable environment of the system.
Another photoelectric phase-discriminated and phase-locked technology is based on the microwave photonic device. The most direct method of the technology is to convert the optical signal into an electrical signal, and then use the electrical phase-locked loop for phase discrimination and phase locking, that is, only add PD to the front stage of the RF mixer, then it is a photoelectric phase discriminator based on the RF mixer. The technology is suitable for the locking of optical signals and electrical signals and the locking between optical signals and has the advantages of simple principle and low implementation cost. However, due to the bandwidth limitation of the RF mixer, it cannot be applied to systems with high frequency or high bandwidth, and the system noise is large.
However, although the existing photon sampling techniques and coherent locking techniques have been widely studied, the sampling methods combining the two have not been studied. Therefore, we propose a coherent photon analog-to-digital conversion method.